[media] v4l: vsp1: Supply frames to the DU continuously

When operating in DU output mode (deep pipeline to the DU through the
LIF), the VSP1 needs to constantly supply frames to the display. To
ensure reuse the last queued buffer instead of returning it to the user.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>